The present invention relates to spherical-like composite particles and an electrophotographic magnetic carrier comprising the spherical-like composite particles, and more particularly, to spherical-like composite particles having a freely controllable coercive force and a high volume resistivity, and an electrophotographic magnetic carrier comprising the spherical-like composite particles.
The spherical-like composite particles according to the present invention can be mainly applied to a developing material for developing an electrostatic latent image, such as an electrophotographic magnetic carrier and an electrophotographic magnetic toner, a wave absorbing material, an electromagnetic shielding material, an ion exchange resin material, a display material, a damping material or the like. Especially, the spherical-like composite particles according to the present invention can be suitably used as the electrophotographic magnetic carrier.
In recent years, as materials having a high performance and novel functions, there have been proposed various composite particles made of different kinds of materials. As one of these composite particles, those composed of inorganic particles and an organic high-molecular weight compound have been variously studied and developed, and put into practice.
In the case where magnetic particles are used as the inorganic particles, the composite particles containing the magnetic particles have been used in various applications such as a developing material for developing a electrostatic latent image, such as an electrophotographic magnetic carrier and an electrophotographic magnetic toner, a wave absorbing material, an electromagnetic shielding material, an ion exchange resin material, a display material or a damping material or the like.
In any of the above-mentioned application fields, the composite particles have been demanded to satisfy such requirements (1) that the content of magnetic particles is as large as possible such that various properties and functions of the magnetic particles can be exhibited to a sufficient extent; (2) that the composite particles are of a spherical shape in order to improve particle properties such as fluidity or packing property; and (3) that the particle size of the composite particles can be controlled in a wide range, especially 1 to 1,000 .mu.m, so as to enable the selection of a desired particle size according to intended applications.
First, there is described the application of the composite particles to a developer for developing an electrostatic latent image. As is known in conventional electrophotographic methods, a photosensitive material made of a photoconductive substance such as selenium, OPC (organic semiconductor) or .alpha.-silicon has been used to form an electrostatic latent image thereon by various means. The thus formed electrostatic latent image is developed using magnetic brush development method or the like by electrostatically attaching thereto a toner having a polarity opposite to that of the latent image, thereby producing a visible toner image.
In the development system, so-called carrier particles are used to impart an appropriate amount of positive or negative charge to a toner by frictional electrification therebetween. In addition, the toner is delivered through a developing sleeve into a developing zone near a surface of the photosensitive material where the latent image is formed, by exerting a magnetic force of a magnet accommodated within the developing sleeve.
In recent years, the electrophotographic methods have been extensively used in copying machines, printers or the like. In these application fields, it has been required that thin lines, small characters, photographs or color original documents are exactly copied or printed. In addition, it has also been required to obtain high-image quality and high-grade quality, and achieve high-speed and continuous image formation. These demands are considered to increase more and more in future.
In general, the development of the electrostatic latent image has been conducted by a magnetic brush development method using a magnetic carrier having a constant coercive force. In this case, it is known that the obtained image quality is varied depending upon a magnitude of coercive force used.
Specifically, in the case where the coercive force is small, high image density can be obtained while definition or gradation of images are deteriorated. On the other hand, in the case where the coercive force is large, the definition or gradation of images are improved while the image density is deteriorated. This is because the small coercive force leads to formation of a magnetic brush with a large height and to a low toner density, while the large coercive force causes formation of a magnetic brush with a small height and a large toner density.
Further, there is a close relationship between coercive force and print speed.
Recently, the print speed of copying machines or printers has been considerably increased as compared to conventional ones. In order to increase the print speed, it is necessary to increase a developing speed of these apparatuses. In order to achieve a high developing speed, it is necessary that the magnetic carrier can be firmly held on the surface of the developing sleeve rotating at a high speed. Therefore, it is preferred that the coercive force of magnetic carrier be large to some extent, because a magnetic brush having a small height and a high toner density can be assured by using such a magnetic carrier having a large coercive force.
In order to satisfy both high image quality and high- speed printing, it is required that the coercive force of magnetic carrier is freely controllable according to the system used.
Further, there has been a recent tendency that the particle size of toner is reduced in order to obtain a high image quality. With the decrease in particle size of the toner, the particle size of magnetic carrier has also been reduced.
However, when the particles sizes of toner and carrier are reduced, there arises a problem that the fluidity of a developer composed of these small particles is deteriorated. Therefore, there has been a demand for a toner and a carrier having a good fluidity.
Hitherto, various attempts have been performed to control a coercive force of the magnetic carrier. For example, there has been proposed an electrophotographic magnetic carrier comprising magnetic particles having a high coercive force and magnetic particles having a low coercive force in combination (Japanese Patent Applications Laid-open Nos. 60-144759(1985) and 60-196777(1985)).
However, the above-mentioned conventional magnetic carrier is in the form of a mixture comprising different kinds of carrier particles having different coercive forces and, therefore, separated into individual groups of carrier particles in a developing device, so that there arise a problem that defects of the carrier particles are exhibited as they are.
Further, in order to solve the above-mentioned problems, in Japanese Patent Application Laid-open No. 2-88429(1990), there has been proposed so-called composite particles made of ferrite particles which contain both magnetic particles having a small coercive force and magnetic particles having a large coercive force.
However, in the case of such composite particles, although the above-mentioned problem concerning the separation of particles into individual groups is solved, there arises another problem that since these particles composed of ferrite solely, have a large specific gravity and exert a large stress onto a toner, the durability of a developer is deteriorated after a long-term use thereof. Further, since the composite particles are of non-spherical shape, the fluidity thereof is unsatisfactory.
Further, in Japanese Patent Application Laid-open No. 6-11906(1994), there has been described a binder-type carrier, i.e., a magnetic carrier containing magnetic particles having a coercive force of not less than 300 Oe and magnetic particles of less than 300 Oe.
More specifically, in Japanese Patent Application Laid-open No. 6-11906(1994), there has been described a magnetic carrier used for a magnetic brush toner/carrier development of an electrostatic charge pattern, comprising a binder resin and fine magnetic pigment particles dispersed in the binder resin, wherein said magnetic pigment particles are in the form of a mixture of a part (A) having a coercive force of not less than 300 Oe and another part (B) having a coercive force of less than 300 Oe, with the weight ratio of the part (A) to the part (B) being in the range of 0.1 to 10.
However, since these particles are of a non-spherical shape due to the production method, the fluidity thereof is deteriorated.
Besides, in Japanese Patent Application Laid-open No. 6-35231(1994), there has been proposed a magnetic substance dispersing-type resin carrier having a composite phase of a spinel structure and a magnetoplumbite structure.
More specifically, in Japanese Patent Application Laid-open No. 6-35231(1994), there has been described a magnetic substance dispersing-type resin carrier comprising a binder resin, and magnetic particles dispersed in the binder resin and having a particle size of 5 to 100 .mu.m, a bulk density of not more than 3.0 g/cm.sup.3, and magnetic properties that the magnetization (.sigma..sub.1000) at a magnetic field of 1,000 Oe is 30 to 150 emu/cm.sup.3 ; the magnetization at a magnetic field of 0 Oe (residual magnetization: .sigma..sub.r) is not less than 25 emu/cm.sup.3 ; and the coercive force is less than 300 Oe, the content of the magnetic particles being 30 to 99% by weight based on the total weight of the carrier.
However, in these particles, the content of particles having a magnetoplumbite structure is smaller than that of particles having a spinel structure, so that the composite particles has a low coercive force. In addition, the volume resistivity of the composite particles is considerably influenced by the weight ratio between two types of particles. Therefore, it is difficult to adjust the volume resistivity to a level as high as required.
As a result of the present inventors' earnest studies, it has been found that by dispersing magnetically hard particles having a coercive force of not less than 500 Oe and magnetically soft particles having a coercive force of less than 500 Oe in a specific amount of a phenol resin binder, in which the ratio of an average particle size of the magnetically hard particles to that of the magnetically soft particles lies in a specific range, the obtained spherical-like composite particles can exhibit a desired coercive force and a desired high volume resistivity, and are suitable as an electrophotographic magnetic carrier. The present invention has been attained on the basis of this finding.